The present invention relates to power cables for high and very high voltage DC.
1. Field of the Invention
The cables to which the present invention applies are cables for 60 kilovolts (kV) to 600 kV or more, and preferably cables for 150 kV or more, operating with DC and having extruded polymeric insulation.
2. Description of the Related Art
Document JP-A-2-18811 discloses a DC power cable comprising a conductive core and extruded polymeric insulation surrounding the core. The insulation is constituted by a mixture of high density polyethylene, low density polyethylene, peroxide, and preferably carbon black in the form of fine particles, having 2% to 20% by weight high density polyethylene and 0.5% to 1.5% by weight of carbon black, and it is cross-linked. The insulation is intended to improve the breakdown characteristics under a DC voltage and under a surge voltage, in particular due to a lightning strike on the cable, compared with the same characteristics for an analogous cable in which the insulation has only one type of polyethylene.
The small quantity of carbon black incorporated in the insulation of that known cable minimizes the risks of breakdown due to defects in the insulation. It gives rise to dielectric losses in the insulation of the DC cable, which losses are of little importance in the absence of defects and when the electric field is weak, but they become excessive and unacceptable when defects are present and the electric field is strong.
Document EP-A-0 539 905 discloses a high voltage DC cable in which the insulating material is made of a thermoplastic rubber having an elastomeric phase and a thermoplastic phase. In a first embodiment of that cable, the thermoplastic rubber can be of the olefin type. In which case, the elastomeric phase is constituted by an ethylene-propylene rubber and the thermoplastic phase is selected from polyethylene and polypropylene. In a second embodiment, the thermoplastic rubber can be of the styrene type. In which case, the elastomeric phase can be hydrogenated and selected from polybudadiene and polyisoprene, and the thermoplastic phase can be constituted by polystyrene. The insulation of that known cable makes it possible to reduce the phenomenon whereby space charge accumulates in the presence of high voltage DC.
An object of the present invention is to make a high and very high voltage DC cable that avoids dielectric losses in the insulation and that presents simultaneously optimized characteristics for withstanding breakdown under a DC voltage and avoiding breakdown under a surge impulse voltage, for a high working voltage and with a quantity of space charge that is minimized in the presence of high voltage DC, so as to provide a cable of very good reliability.
The invention provides a high or very high voltage DC cable comprising a conductive core and extruded polymeric insulation made of a styrene-containing material, wherein said material is constituted by a mixture of polyethylene, a hydrogenated block copolymer of styrene selected from copolymers of styrene and butadiene and of styrene and isoprene, present at a styrene content by mass lying in the range 11% to 18%, and it is not cross-linked.
By means of this insulation, the working voltage under steady conditions is particularly high and simultaneously the risk of breakdown is made very low, thereby increasing reliability of the cable.
Advantageously, the mass concentration of styrene in said mixture is selected to lie in the range 11.5% to 16%.
According to an additional feature, said cable includes an inner semiconductive screen between said conductive core and said insulation, and an outer semiconductive screen around said insulation, both screens being constituted by a polymeric matrix which is selected to be of the same nature as said insulation, which contains a conductive filler, and which is not cross-linked.